Spectroscopy is an important analytical tool for measuring various properties of an object. Recently, terahertz (THz) spectrometry has been developed for analyzing these properties. THz spectroscopy uses visible to near-infrared laser pulses, each lasting only about ten to several hundred femtoseconds, in order to generate electromagnetic pulses (T-rays), which last about a picosecond. These T-rays are then transmitted through an object using an imaging system of lenses and mirrors. Based on the changes in the T-rays as they pass through the object, information may be determined about certain properties of the object. For example, a THz spectrometer may be used in this way to ascertain the caliper, moisture and/or basis weight of paper.
The precision of amplitude and phase measurements in spectroscopy, including THz spectroscopy, is often limited by noise in the system. For example, fluctuations in environmental parameters, such as temperature changes, mechanical vibrations or changes in air composition, may result in pulses that have traveled through the same material reaching the detector at slightly different times and with slightly different amplitudes. Conventional methods of attempting to overcome these errors include attempting to control the environmental parameters. For example, systems may be implemented to minimize changes in temperature and air composition and to minimize vibrations in the system. However, these systems may not be capable of controlling these parameters enough to produce results with a desired degree of accuracy.